The field in which the invention is utilized includes many examples of structures utilizing a flapper which is positioned relative to a nozzle or a pair of opposed nozzles to control the flow of fluid through such nozzles to generate a control pressure or a control pressure differential upstream of the nozzle or nozzles. Generally these structures include a control input such as an electric force motor or a thermostatic bi-metal element which modulates the position of the flapper relative to the nozzle or nozzles. As the need for faster acting and more accurate controls has developed, attempts have been made to reduce centering forces, such as spring forces, on the flapper so that the input force and nozzle flow forces generate a larger percentage of the total force applied to the flapper to position the flapper relative to the nozzles. The spring mass system determines a natural harmonic frequency at which the flapper may vibrate under certain conditions. Such harmonic vibration of the flapper generates an annoying buzz and furthermore reduces response time for stabilized control and accuracy of the flapper movement relative to the nozzles.
Attempts have been made to damp the movement of the flapper particularly in the range of harmonic frequencies so as to reduce these adverse effects. One such attempt is represented by U.S. patent Hedlund, U.S. Pat. No. 3,426,970 , issued Feb. 11, 1969, wherein a particular physical relationship between the nozzle end face and the flapper is provided to reduce vibratory motion of the flapper. Such a flapper nozzle end face parameter design however is maximized for damping action and does not permit the nozzle to be designed to the ultimate parameters relative to flow control. Testing has shown that the most effective nozzle flapper interface is defined by a sharp edge whereas the damping feature of Hedlund requires a broad flapper/nozzle end wall interface. The squeeze film damper of the present invention is spaced from the flapper/nozzle interface but still utilizes the fluid flow from the nozzle to provide fluid for the damping action.
U.S. patent Lloyd, U.S. Pat. No. 3, 009,447, issued Nov. 21, 1961 teaches an electric force motor pressure control wherein a spring centered flapper is positioned between two opposed nozzles to provide a control pressure differential upstream of the nozzle which acts as the pilot control pressure. There is no disclosure of damping action generated between the flapper and the flat end faces of the opposed nozzle and this interface would have the same problems of interfering with maximizing the control of nozzle flow as discussed above with respect to Hedlund. Furthermore Lloyd utilizes a complicated pressure and velocity feedback system including an auxiliary load mass to provide refined system control.
One manufacturer utilizes a restriction in the exhaust passageway to aid in damping undesirable vibrations in an arrangement including an electric force motor controlling a flapper relative to a nozzle. It is believed that such an arrangement is relatively ineffective in damping harmonic vibration and increases the pressure downstream of the nozzles. This reduces the available pressure drop across the nozzles which in turn reduces the allowable or permissible pressure differential that can be generated.
While various studies of squeeze film damping have been conducted, for example, the June 1966 Journal of Basic Engineering article entitled "A Study of Squeeze-Film Damping", such studies are generally directed to flat plate damping and not directed to the specific damping structure of the present invention or the utilization thereof in an arrangement where a flapper controls nozzle flow.